The replacement of X-ray screen/film systems by computed radiography (CR) started about 10 to 15 years ago. Until recently, the conversion was limited to general radiography. Now, the CR companies are introducing mammo CR as well. As is known in different X-ray imaging applications different X-spectra and different X-ray doses are used. Mammographic imaging in particular differs from other imaging applications, which are considered under the common name “general radiography”.
As a consequence of differences in spectra and dose use of specific phosphor screens for mammography is required, that are quite different from the general radiography screens. As a further consequence other factors are responsible for image quality in mammography than in general radiography.
Typical X-ray exposure conditions in general radiography are the so-called RQA-5 conditions [IEC(6)1267:1994]. The spectrum is generated by an X-ray source having a tungsten anode at a 70 kVp setting. The spectrum is filtered by an internal and an external aluminum filter having a thickness of 2.5 mm and 21 mm respectively. RQA-5 exposure conditions correspond to an “Al half-value thickness” of 7.1 mm. The average energy of the X-ray quanta is about 55 keV.
For a typical general radiography dose of 0.3 mR the number of X-ray quanta to which the phosphor screen is exposed under RQA-5 conditions is 8.6 104 quanta/mm2. The number of quanta absorbed by the phosphor screen is typically ca. 2.5 104 quanta/mm2.
A typical mammo spectrum is generated by an X-ray source having a Mo anode at a 28 kVp setting. An internal Mo filter with a thickness of 0.03 mm is typically used. If the X-ray spectrum is additionally filtered by 42 mm of PMMA a typical spectrum is generated that reaches the phosphor plate in mammographic imaging. In this case, the average energy of the X-ray quanta is ca. 20 keV.
For a typical mammography dose of 10 mR the number of X-ray quanta to which the phosphor screen is exposed under the above described mammoconditions is: 4.7 105 quanta/mm2 and the number of absorbed quanta is of the order of 3×105 to 4×105.
Hence, the number of quanta used to make an image is more than 10 times higher in mammography than in general radiography. As a consequence, quantum noise, due to fluctuations in the number of X-ray quanta absorbed per pixel is relatively large in general radiography and less important in mammography. In general radiography computed radiography (CR) quantum noise is the only important noise source, which means that signal-to-noise ratio is primarily determined by the X-ray absorption and the sensitivity of the phosphor screen. Analysis of noise components of various imaging plates, wherein CR system noise is classified in quantum noise and fixed noise, has e.g. been described in Med.Imaging 2004: Physics of Medical Imaging, p. 686, Proceedings of SPIE, Vol. 5368. Since quantum noise is smaller in mammography another noise source, screen-structure noise, has a significant contribution to the total noise in the image as well.
Therefore, contrary to what is the case in general radiography, screen-structure noise must be reduced to a minimum in order to have a good phosphor screen for mammographic imaging, i.e. the phosphor screen must be made as homogeneous as possible.
U.S. Pat. No. 6,383,412 relates to a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor, a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor having a tetradecahedral structure in particular, a process for preparing the phosphor, and a radiographic image conversion panel using the phosphor. That phosphor has a grain size median diameter (Dm) of 1 to 10 μm, a standard deviation on the average grain size of 50% or less for grains having a grain aspect ratio within the range of from 1.0 to 2.0. An object attained therewith was to provide a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor capable of producing high-quality images having a very high sharpness and exhibiting other excellent emission characteristics, high sensitivity and erasure characteristics in particular, when used in radiographic image recording and reproduction.
Apart from those image quality properties, noise measurements have been performed e.g. in “Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment”, Vol. 430, issues 2-3, July 1999, p. 559-569. Therein the phosphor screens are brought in close contact with a film (AGFA Scopix LT2B) sensitive to their emission spectrum (red). The above configuration was irradiated with a mammography X-ray unit (molybdenum target tube and 30 kVp X-ray spectrum filtered by a 51 mm plexiglass). The exposure was 6.32 mR. This value is among the lowest values reported for NPS measurements in 30 kVp as taught in Med. Phys. 12 (1985), p. 32; Med. Phys. 19 (1992), p. 449 and Radiology 145 (1982), p. 815. Furthermore it has been said therein that as a preferred technique the sedimentation technique, used for screen preparation, results in uniform distribution of phosphor grains. Contribution of the screen structure noise to NPS for the above mentioned exposure value can thus be considered small as compared to the quantum noise pattern. Structure noise properties of granular phosphors used in X-ray imaging detectors have further been studied in terms of a noise transfer function, NTF as disclosed in “Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment” Vol. 490, issue 3 (2002), p. 614-629. That study has been performed in high-exposure conditions where the contribution of structure noise to total screen noise is considerable. An analytical model, based on the cascaded linear systems methodology presented in the literature, has been developed, wherein that model takes into account quantum noise and structure noise. Furthermore, it considers the effect of the K X-rays reabsorption on the phosphor material and the effect of screen thickness on the NTF. The model was validated against experimental results obtained by a set of Zn2SiO4:Mn phosphor screens prepared by sedimentation. The model may be used to evaluate the effect of screen thickness and the effect of the characteristic X-rays on NTF in high-exposure conditions where structure noise is considerable.
The effect of screen thickness is particularly important in high-exposure conditions, where the screen structure noise is dominant. Screen structure noise is attributed to fluctuations of the absorbed X-ray quanta due to the inhomogeneities in the phosphor coating as has been described by Barnes in Med. Phys. 9(1982), p. 656. This component is negligible in quantum-limited (i.e. low-exposure) conditions, but in higher exposure conditions it should be taken into consideration. Screen noise is evaluated in terms of either NPS (also called Wiener spectrum), or Noise Transfer Function, NTF Med. Phys. 17 (1990), p. 894. Since quantum and structure noise are statistically independent and uncorrelated, total screen NPS equals the sum of the corresponding NPS of quantum noise and screen-structure noise.
It is moreover general knowledge that sharper images with less noise are obtained with phosphor particles of smaller mean or average particle size, but otherwise, it is well-known that light emission efficiency declines with decreasing particle size. Thus, the optimum mean particle size for a given application is a compromise between imaging speed and image sharpness desired. Until now preferred average grain sizes of the phosphor particles are in the range of 2 to 30 μm and more preferably in the range of 2 to 20 μm , in particular for BaFBr:Eu type phosphors.
Dedicated storage phosphor screens are thus required for mammographic applications, more particularly as a higher sharpness is needed. Moreover since the number of X-ray quanta contributing to the image is higher than in general radiography, quantum noise is reduced and screen-structure noise has a significant contribution. Hence, screens must be developed with a higher homogeneity, i.e. the pixel-to-pixel sensitivity fluctuation should be reduced as much as possible.